The invention relates to a process for pretreating a very acid natural gas containing a substantial amount of acid compounds such as hydrogen sulfide (H2S) and carbon dioxide (CO2).
The work done by the applicant had allowed to propose, in patent EP-0,665,046, a process intended to eliminate a substantial amount of the acid compounds present in the natural gas at the well outlet, a process whose simplicity enabled ready implementation with a minor investment. According to the process, the initial natural gas is contacted in a cyclone type enclosure with a liquid rich in acid compounds so as to obtain, on the one hand, at the top of the cyclone enclosure, a gaseous fraction depleted in acid compounds and on the other hand, at the bottom of the enclosure, a liquid phase containing the major part of the acid compounds and water. The liquid phase collected at the bottom of the enclosure is reinjected into a well nearing depletion. The gaseous fraction obtained at the top of the cyclone enclosure is cooled to a low temperature (down to xe2x88x9230xc2x0 C.), then sent to a separating drum to obtain, on the one hand, a gas freed of acid compounds and, on the other, a condensate rich in acid compounds which is recycled to the cyclone enclosure.
However, the process described in patent EP-0,665,046 comprises drawbacks:
1) The presence of water in the gaseous fraction cooled to a low temperature is likely to lead, throughout the circuit, to the formation of solid hydrates that may eventually clog the pipes or damage the constituent elements of the device used. This is why the process described in patent EP-0,665,046 recommended the use of an anti-hydrate, preferably methanol, to prevent hydrate formation during cooling of the gaseous fraction from the cyclone enclosure.
2) An appreciable amount of hydrocarbons is carried along with the liquid phase collected at the bottom of the cyclone enclosure. The loss of hydrocarbons carried along with the liquid at the bottom of the cyclone enclosure can reach 10% of the amount of gas treated.
The object of the present invention is notably to overcome the aforementioned drawbacks.
It has been discovered by the applicant that it is possible, under suitable thermodynamic conditions, to concentrate the initial natural gas with methane while removing the majority of the acid gases and substantially all of the water it contains. The expression substantially all of the water means that the amount of water present in the final gas is less than 50 ppm by mole, preferably less than 10 ppm by mole, and more preferably less than 5 ppm by mole. The present invention proposes replacing the cyclone type contacting enclosure by several contacting zones, each contacting zone being operated under predetermined pressure and temperature conditions.
The present invention also proposes using various means for cooling the gaseous fraction obtained at the top of the cyclone enclosure.
The present invention proposes a process for pretreating a natural gas under pressure containing hydrocarbons, at least one of the acid compounds H2S and CO2 and water, the process comprising the following stages:
a) cooling the natural gas so as to produce a liquid phase and a gas phase,
b) contacting, in a first contact zone, the gas phase obtained in stage a) with a liquid phase obtained in stage c) to produce a gas phase and a liquid phase,
c) contacting, in a second contact zone, the gas phase obtained in stage b) with a liquid phase obtained in stage d) to produce a gas phase and a liquid phase,
d) cooling the gas phase obtained in stage c) to produce a liquid phase and a gas phase.
In stage d) of the process according to the invention, the gas phase obtained in stage c) can be cooled by means of a heat exchanger and/or of an expander.
The process according to the invention can also comprise a stage of:
e) cooling the gas phase obtained in stage d) by means of an expander to produce a gas phase and a liquid phase which is recycled to stage c).
If the process according to the invention uses an expander, it can comprise a stage of:
f) compressing at least one of the gas phases obtained in stage d) and stage e) using the energy recovered from the expander.
In stage d) of the process according to the invention, the gas phase obtained in stage c) can be cooled by means of a venturi neck, said liquid phase being discharged in the vicinity of the venturi neck and said gas phase being collected at the outlet of the divergent tube of the venturi neck. The liquid phase discharged in the vicinity of the venturi neck can be cooled to produce the liquid recycled to stage c) and a gas phase.
The gas phases obtained in stages d) and e) can be used to cool the gas phase obtained in stage c) and/or to cool the natural gas in stage a).
The liquid obtained in stage b) can be heated in the first contact zone.
In stage a) of the process according to the invention, the natural gas can be at a pressure of 8 MPa and at a temperature above 15xc2x0 C.
The liquids obtained in stages a) and b) can be injected into a well.
According to the present invention, after treating the natural gas from the production well, a final gas containing the major part of the hydrocarbons contained in the gas before treatment is recovered. What is referred to as the major part of the hydrocarbons is at least 90% hydrocarbons, preferably at least 95% hydrocarbons and most preferably at least 97% hydrocarbons in relation to the hydrocarbons contained in the gas before treatment.
The present invention advantageously allows to avoid using an anti-hydrate such as methanol, whose transportation, use and regeneration are generally expensive, complex and which is dangerous to handle.